1. Field of the invention
This invention relates to an incandescent lamp and its use, in particular, to a novel incandescent lamp operable at an elevated voltage which encloses a filling composition consisting of krypton gas and nitrogen gas, as well as to its use.
2. Description of the prior art
Generally, the elevation of filament temperature in incandescent lamps improves their luminous efficiency. Such an elevation however accelerates the vaporization of tungsten and the vaporized tungsten attaches and blackens on the inside surface of glass envelope to reduce luminous efficiency and life expectancy. To suppress the vaporization of tungsten, incandescent lamps enclose inert gas such as nitrogen gas and/or rare gas, for example, argon gas, krypton gas and xenon gas. It is well known that, in this case, the larger the molecular or atomic weight of the enclosed inert or rare gas, the heat loss on glass envelope becomes much less, thus enabling an elevated filament temperature.
Also is known that krypton gas has a relatively large atomic weight among these inert or rare gases and exhibits satisfactory luminous characteristics and extended life expectancy when enclosed in incandescent lamps. Krypton however has the disadvantage that when excessively enclosed in glass envelope, its low ionization potential is causative of switch-on arc discharge which may accelerate the burnout of filament, therefore, in conventional krypton lamps, about 5-10% by volume of nitrogen gas is used in combination to suppress arc discharge and the operating voltage is set around that of standard ac line, in particular, 100-110 V. Although, of course, the use of an elevated nitrogen gas content and other one or more rare gases with a higher ionization potential and/or better luminous characteristics such as argon gas and xenon gas has been proposed, these proposals are effective in improving arc starting voltage but have the disadvantage that they hinder the elicitation of superior luminous characteristics inherent to krypton gas and low-cost production of incandescent lamps.
Thus, there have been available no low-cost incandescent lamps which enclose about 80-95% by volume of krypton gas to elicit its inherent desirable properties but hardly cause arc discharge when operated at a relatively high voltage, in particular, about 200-275 V.